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Box PI is your only source for the fattest rear bar (1”). Handling RPO cars got a smaller bar that’s not too hard to find that’s better than nothing. In 1990, the frame changed and you can use a rear aero bar IIRC. Look up above your rear axle for two frame brackets with single holes in the middle, forward. Late 89 might be close enough but I doubt it.

You can pick up an addco front bar from summit or similar. It will bolt right up. I had one installed and removed it because it was too wide. At full lock the tires would rub it.

You can also install aero front bars. Those normally get different end links and attach to the spindles, though I recall some peeps just using the stock box end links on them and everything working out just fine.

I have one on my car attached to the spindles as I have upgraded to big brakes.

I was having a conversation about this with a friend who is very knowledgeable about automotive engineering, and other engineering I suppose. Apparently it is, above all else, the diameter of the sway bar that dictates its effectiveness, and as he explained it it does make sense. The twisting action applied to the bar will be better resisted the further away from the center line the outer surface of the bar is (think of it like increasing the length of a lever to give you more leverage).

He said in almost any case, if you can have a bar of a particular dimension that is solid, or you can have one of a somewhat bigger dimension that is hollow, go with the bigger diameter one to see the most effective benefit.

I can't say I can find any information to the contrary. There are obviously strength considerations; a skinny solid bar will perform equivalent to a slightly larger hollow bar. In some suspensions, space is a consideration, so saving space by using a skinnier solid bar makes sense. Since we've generally got acres of room with the only pinch point being where the lower rad hose wants to be, a huge bar isn't really an issue.

I measured my front bar lastnight and its 1-3/16" so it either has a tow package or had already been upgraded. I only have one hole in the rear control arms so if I ever find a bar that fits ( which seems impossible to find) i have no problem drilling a second hole

I didn't have to fuck with my rear control arms on either '90 to put the cop sway bar on them. Maybe this is an earlier box thing?

Both of my '91s only have one hole for a sway bar (build dates 3/90 and 11/90), I'd have to bolt the sway bar on at the existing hole and drill the second hole. I'm pretty sure I have aero mount plates for an aero sway bar though so maybe that was the reason I didn't get the other box sway bar hole in my control arms.